This invention relates generally to variable stator vane outer trunnion bushings used in gas turbine engines and more particularly to a variable stator vane outer trunnion bushing that eliminates the galvanic corrosion potential of conventional bushings.
A typical gas turbine engine compressor includes several rows or stages of compressor stator vanes and corresponding rows or stages of compressor rotor blades therebetween. As ambient air flows through each succeeding compressor stage during operation, it is successively compressed for providing compressed air to a combustor located downstream therefrom wherein it is mixed with fuel and ignited for generating hot combustion gases that power the engine.
To improve the overall operation of the compressor, several compressor stator vanes are rotatively mounted to allow each vane to rotate around its longitudinal or radial axis to adjust the angular orientation of the vane relative to the airflow thereover. Such variable stator vanes include an integral outer trunnion disposed in a complementary mounting boss in the stator casing for allowing rotation of the vane relative to the casing. A lever arm is fixedly joined to a coaxial stem extending outwardly from the vane trunnion. The distal end of the lever arm is operatively joined to an actuation ring that controls the angle of the vane. All of the vane lever arms in a single row are joined to a common actuation ring for ensuring that all of the variable vanes are positioned relative to the airflow in the compressor stage at the same angular orientation.
Variable stator vane assemblies include a bushing disposed between the outer trunnion and the casing mounting boss to decrease the coefficient of friction therebetween and prevent wear of the trunnion and casing. These bushings are generally made by compression molding techniques using high temperature polyimide resin and braided graphite composite laminate. Often the bushings are designed with very small tolerances that require finish grinding to bring the dimensional tolerances into acceptance. The finish grinding can result in the exposure of graphite fibers, which, upon installation of the bushing into the mounting boss, come into direct contact with the bare steel surface thereof.
During operation of the engine, corrosive species, such as salt water, can infiltrate crevices and gaps between the bushing and the mounting boss, thereby creating a galvanic cell between these two dissimilar metals. This can lead to the formation of galvanic species and oxides such as rust between the bushing and the mounting boss. Excessive rust formation will squeeze the bushing, causing it to be reduced in size to the point that it will pinch the trunnion, thereby preventing the desired rotational operation of the variable stator vane assembly.
Accordingly, there is a need for variable stator vane outer trunnion bushing that eliminates the galvanic corrosion potential of conventional bushings.